-ir stem change (Indefinido / Gerundio / Subjuntivo)
An -ir verb that changes its stem in the presente changes it in three more places, one step only — e → i, o → u: the third person of the indefinido (pidió, durmió), the gerundio (pidiendo, durmiendo), and nosotros and vosotros of the presente de subjuntivo (pidamos, durmamos).
The card. The back shows one form of each: the él form of the Pretérito Indefinido, the Gerundio, and the nosotros form of the Presente de Subjuntivo, each labelled. The book opens the verb's conjugation at those three.
The deck. Every -ir verb in your selection with a stem change. The stem-changing verbs in -ar and -er keep their stem outside the presente, so they are not in it. About one card in five is a regular verb — twenty irregulars bring five — drawn afresh each time you open the drill, so you have to decide whether the verb departs at all.
Grading and the session work as in Future stem (Futuro / Condicional), and each graded card is recorded once for every form on its back.